An end shield is a standard structure that can be included in electrical machines. The end shield closes the opening at an end of a frame of an electrical machine through which a stator can be installed into the frame. The end shield can also serve as an attachment base for bearings supporting the rotor. It can transfer the rotor's supporting forces to the frame of the electrical machine.
The end shield can protect electrical and rotating parts of the electrical machine and serve as a structure supporting the bearing. These can impose several functional specifications, on the end shield such as stiffness and a small axial space. Furthermore, the end shield should be competitive in terms of manufacturing costs.
A purpose of the end shield can be to support the bearing, which in turn can support the rotor. Stiff support of the rotor can enable increased specified rotational speeds and reduce vibration levels of the bearings. Stiff support of the rotor can be achieved through radial support. The stiffness of radial support can be affected by radial and rotational stiffness of the end shield. The latter refers to the shield's ability to receive a bending moment, represented by a vector in a same plane as the shield and transmitted by the bearing, without substantial rotation.
Axial stiffness of the end shield and the frame end is desirable in vertical machines in which a mass of the rotor and an axial load from the machine can be transferred to the frame of the electrical machine through the end shield and the frame end. The axial stiffness of the end shield and the frame end can also affect a natural frequency of axial mode of the frame end. The frequency, in turn, is closely related to vibration levels of the bearings.
A small axial space specification is desirable for the end shield. It can be desirable to achieve the shortest possible bearing span in relation to the axial space reserved for active components (for example, stator, windings, stator connection and air insulation). The space specification of the end shield can be measured by a distance between a plane perpendicular to the axial direction touching an air insulation space and a parallel plane intersecting a middle plane of the bearing. The vertical plane, perpendicular to the axial direction and touching the air insulation space, is determined to be in the location where the air insulation space is completely on the stator side. The axial middle plane of the bearing can be the symmetry plane of the rolling or sliding elements. The space specification of the end shield is considered for two-pole stiff shaft electrical machines in which a specified rotational speed is to be kept above the operating rotational speed.
Known end shields of large electrical machines have been manufactured from thick steel sheet by machining. An advantage of a machined steel shield is its economic efficiency, as a factor affecting the price is the thickness of the steel shield. However, a bending stiffness of a steel sheet of uniform thickness is poor in relation to its weight. Furthermore, a local stiffness of the bearing attachment is determined by the thickness of the end shield. Thus, general bending stiffness and the local stiffness of the attachment can be improved, by increasing the thickness of the end shield. On the other hand, increasing the thickness of the end shield can increase the material costs as well as the axial space requirement of the end shield within the electrical machine.
The bending stiffness of a steel shield can also be increased by reinforcements, for example by welding external reinforcements onto the shield. This introduces new stages of end shield manufacture and creates more costs. Furthermore, in the placement of the reinforcements, the functional specification for the end shield—as well as factors affecting its appearance—should be considered.
Cast shields can be used in small electrical machines. A cast shield enables the optimization of the features by design. Heat transfer has a role in the design of a cast shield. The support stiffness for small electrical machines does not usually cause problems. Cast shields are rare with larger electrical machines due to the short production runs.